Surface acoustic wave (SAW) devices have been in extensive use as bandpass filters in communication equipments such as a cellular phone and the like. With the enhancement of the performance of cellular phone and the like, there is an increasing demand to enhance the performance of the filters using the surface acoustic wave devices.
In the surface acoustic wave device, however, there is a problem of the passband shifts due to temperature changes. In particular, lithium niobate and lithium tantalate frequently used have high electromechanical coupling coefficients, and thus provide an advantage in achieving broad-band filtering characteristics. However, lithium niobate and lithium tantalite are of inferior temperature stability.
For example, in lithium tantalate, the temperature coefficient of frequency change is −35 ppm/° C. and the frequency varies greatly in the temperature range of −30° C. to +85° C. Therefore, it is necessary to lower the temperature coefficient of the frequency change.
Japanese Patent Publication No. 2001-53579A describes a device fabricated by bonding a SAW propagation substrate and a supporting substrate together with an organic thin film layer. The propagation substrate is a 30-μm-thick lithium tantalate substrate, for example, and is bonded to a 300-μm-thick glass substrate with a 15-μm-thick organic adhesive.
Japanese Patent Publication No. 2006-42008A describes a SAW device fabricated by laminating a lithium tantalate substrate (125 μm in thickness) and a quartz glass substrate (125 μm in thickness) together with an adhesive.
Japanese Patent Publication No. Hei 06-326553A, Japanese Patent No. 3774782B and U.S. Pat. No. 7,105,980B also describe SAW devices fabricated by bonding a SAW propagation substrate and a supporting substrate together.
Further, according to Japanese Patent Publication No. 2005-229455A, oxidized layers of a thickness of 0.1 to 40 μm are formed on both main faces of a silicon supporting substrate, and an piezoelectric substrate is then bonded to the supporting substrate to produce a SAW device. The oxidized layer of silicon is indispensable for reducing the warping of the thus obtained composite piezoelectric substrate 1.